Coating compositions, more particularly polyurethane coatings produced by reacting isocyanate resins with polyol resins, are well known. Such coatings are known to feature a range of outstanding properties, including high durability and chemical stability. Their glamour properties make them outstandingly suitable for automotive refinish, utility-vehicle and automobile-repair applications, and also for industrial applications. In comparison to aqueous compositions, they feature substantially better drying times and improved corrosion protection. With these coatings, however, the polyol, owing to the formation of hydrogen bonds and owing to the molecular weight of the main polymer chain, makes the greatest contribution to the overall viscosity of the coating, thereby limiting the freedom to incorporate other substances at the formulation stage and to meet the specifications relating to the application solids content. The reduction of the molecular weight of the polyol leads ultimately to slow network development, unless considerable quantities of energy are introduced.
Although it is possible by using more solvents to reduce the overall viscosity of the coating and so to accelerate the development of a network, the use of large quantities of solvents is environmentally objectionable. More particularly, coating compositions with high fractions of volatile organic compounds (VOCs), more particularly solvents, fail to match the provisions of directives which lay down a maximum VOC content. In the future it will be necessary to realize very low solvent contents or VOC contents, of not more than 280 g/l.
The properties of high-solids, polyol-containing polyurethane coatings can also be improved through the use of reactive diluents. One application of this technique has been shown by U.S. Pat. No. 5,214,086. It describes the use of oligoaldimines, oligoketimines, and hindered oligoamines for modifying polyol compositions, this modification leading in total to a reduction in the viscosities of the mixtures together with the maintenance, and often improvement, of the development of a network, and of the physical properties. On aluminum and on multimaterial substrates, however, the substrate being either in the exposed state or in the unexposed state, the adhesion of these coating compositions is unsatisfactory.
Adhesion to aluminum is typically obtained in another way, through the use of chromate-containing fillers (Lückert, Pigment+Füllstofftabellen, Lückert Verlag, ISBN 3-927342-03-3; Glasurithandbuch 1984, Vincentz Verlag, ISBN 3-87870-192-6; Peter Volk in “Metalloberfläche”, special issue May 2006). On account of the environmental burden, however, the intention is that the desired properties should be achieved without the use of chromate.
It is an object of the invention, then, to develop high-solids coating compositions which comprise chromate-free fillers and pigments and which exhibit good adhesion to aluminum, steel and/or multimaterial substrates in both the exposed and the unexposed state. On weathering, the coating compositions ought to continue to exhibit good properties. Finally, they ought to exhibit properties such as the mediation of sufficient corrosion protection and satisfactory drying times. Depending on the field of use, the coatings must be abradable after drying or readily overcoatable, for wet-on-wet applications, with conventional and aqueous coating materials.